The pitch of an hydraulic, variable speed propeller, driven by a gas turbine engine, is typically controlled by a governor directly driven by the output shaft of a gearbox associated with the engine. In normal operation, the governor is responsive to the speed of the output shaft and modulates the pressure and volume of a fluid, such as oil, to vary the pitch of the propeller's blades and maintain a constant speed commensurate with the setting of the governor. However, in the event that the mechanism for driving the governor should fail, the governor would sense a drop in speed and reduce the pitch of the propeller's blades and hence its ability to absorb power from the engine. Since a gas turbine engine can accelerate very rapidly, there would be a substantial danger that the engine would overspeed to destruction before the operator of the engine could take corrective action.
One way to prevent overspeeding is to provide a second governor to sense the overspeed condition and assume control. The second governor would then increase the pitch of the propeller's blades by venting oil from the propeller control mechanism thereby limiting the maximum speed of the engine. However, if both the propeller governor and the overspeed governor are driven by the same drive mechanism, a failure of that mechanism would render both governors inoperative. To cope with such an occurrence, the present invention provides means for immediately reducing the pressure and volume of the the fluid being supplied to the propeller control mechanism thereby increasing the pitch of its blades and preventing the engine from destructive overspeeding.